1. Field of the Invention
This invention relates, generally, to optics. More particularly, it relates to scribing thin films in photovoltaic cells.
2. Description of the Prior Art
Conventional scribing of thin films used in solar cells uses focused Gaussian or Gaussian-like laser beams. A round Gaussian laser beam is passed through a focusing lens. Typically, a singlet, best form lens, or a doublet is used, depending upon how tightly the user wants to focus. The price of the optic increases with increasing complexity of the focusing objective.
The substrate to be scribed is placed at the focus of the lens. Thin films ablate from the substrate after the intensity of the focused laser beam reaches a particular threshold. The laser energy absorbed prior to ablation threshold goes into the material as heat, causing thermal damage to the thin film and surrounding area.
Moreover, laser beams are predominately round. Accordingly, a large overlap of consecutive spots is required to minimize scalloping at the edge of the scribe line. This large overlaps slows down the scribing process considerably.
The homogenization of a laser beam can correct the non-steep side profile of a Gaussian distribution. However, homogenization is difficult to implement for scribing applications because the optics used in such scribing systems tend to be flying optics; this precludes using imaging-based homogenized laser beams.
A free form phase shifting optical device can produce a top hat profile that would otherwise be desirable. However, such devices cannot tolerate variation to the input laser beam or beam misalignment. Accordingly, they have not found much use in production systems.
Focused laser beams that are predominately Gaussian or Gaussian-like in shape are not ideal where a steep energy transition is needed to cleanly remove a thin film.
A free form phase shifting optical device such as disclosed in U.S. Pat. No. 6,295,168 transforms a Gaussian laser beam into a top-hat homogenized field but is difficult to implement in production due to the stringent requirements of an extremely stable laser beam and its alignment into the optical system.
A laser beam homogenizer such as disclosed in U.S. Pat. No. 6,621,639 inverts a small section of a Gaussian-like beam for better homogeneity after it passes through a lens array homogenizer. This approach is an imaged based solution limited to lasers with low spatial coherence and does not lend itself to systems that require a flying optics head.
U.S. Pat. No. 6,975,458 discloses an off-axis grating that transforms a Gaussian distribution to create a specific shape in the far field. This approach requires expensive optics and does not have high diffraction efficiency. Moreover, it is usually implemented as an imaging based system which is not suitable for flying head optics.
U.S. Pat. No. 6,697,181 discloses an apparatus that creates an inverted Gaussian shape at a scan focus using special transmission plates and a pyramid type prism. It is not a collimated solution and is therefore not practical for a scribing system.
U.S. Pat. No. 5,798,877 discloses rectangular prisms that break up and reposition a laser beam into two components in an effort to improve the symmetry of a diode array. Several complex prisms are used to accomplish the task.
An inexpensive but effective method for scribing a square spot instead of a conventional round spot is needed. This would eliminate the need for a large overlap of consecutive round spots and would eliminate the scalloping at the edge of the scribe line. It would also speed up the scribing process considerably.
However, in view of the prior art taken as a whole at the time the present invention was made, it was not obvious to those of ordinary skill how the identified need could be fulfilled.